1. Field of the Invention
This invention relates to a process for dewaxing hydrocarbon feedstocks such as distillate fuel oils and gas oils by isomerizing the waxy components, such as straight and lightly branched chain paraffins, over catalyst comprising a siliceous zeolite having been prepared by the method comprising providing a boron-containing zeolite Beta, treating the zeolite with silicon tetrachloride at a temperature and for a time sufficient to replace boron with silicon, and recovering the siliceous zeolite have reduced boron content but substantially preserved initial aluminum content.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline silicates having a definite crystalline structure within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total non-silicon lattice element, e.g. aluminum, and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing, for example, aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the non-silicon lattice element, e.g. aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given crystalline silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic crystalline silicates. These silicates have come to be designated by convenient symbols, as illustrated by zeolite ZSM-5 (U.S. Pat. No. 3,702,886).
The use of certain zeolites as catalyst components is taught in U.S. Pat. No. 4,305,808, for example.
The silica-to-alumina ratio of a given zeolite is often variable; for example, zeolite X (U.S. Pat. No. 2,882,244) can be synthesized with a silica-to-alumina ratio of from 2 to 3; zeolite Y (U.S. Pat. No. 3,130,007) from 3 to about 6. In some zeolites, the upper limit of silica-to-alumina ratio is virtually unbounded. Zeolite ZSM-5 is one such material wherein the silica-to-alumina ratio is at least 5. U.S. Pat. No. 3,941,871 discloses a crystalline metal organo silicate essentially free of aluminum and exhibiting an X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724; 4,073,865 and 4,104,294 describe microporous crystalline silicas or organo silicates wherein the aluminum content present is at impurity levels. Zeolite Beta is described in U.S. Pat. No. 3,308,069, the contents of which are entirely incorporated herein by reference.
The use of zeolite Beta catalyst for dewaxing hydrocarbon feedstocks such as distillate fuel oils by isomerization is taught in U.S. Pat. Nos. 4,419,220 and 4,501,926, each incorporated herein by reference in their entirety.
U.S. Pat. No. 4,486,296 teaches hydrodewaxing and hydrocracking of hydrocarbon feedstock over a three-component catalyst including zeolite Beta. Dewaxing paraffin-containing hydrocarbon feedstock involving a hydrotreating step ahead of the dewaxing step over zeolite Beta catalyst is disclosed in U.S. Pat. No. 4,518,485. U.S. Pat. No. 4,481,104 teaches distillate selective hydrocracking using large pore, high silica, low acidity catalyst, e.g. zeolite Beta catalyst. Hydrocracking C.sub.5.sup.+ naphthas over catalyst comprising zeolite Beta is taught in U.S. Pat. No. 3,923,641. A dewaxing process using a noble metal/zeolite Beta catalyst followed by a base metal/zeolite Beta catalyst is taught in U.S. Pat. No. 4,554,065. U.S. Pat. No. 4,541,919 teaches a dewaxing process using large pore zeolite catalyst such as zeolite Beta which has been coke selectivated; and U.S. Pat. No. 4,435,275 teaches a moderate pressure hydrocracking process which may use catalyst comprising zeolite Beta for producing low pour point distillates.